1. Field of the Invention
The invention concerns digital telephone sets, namely telephone sets designed to transmit or receive speech signals, in the form of a digital signal, and not an analog one, on a telephone line.
2. Description of the Prior Art
These sets have an analog-digital and digital-analog conversion integrated circuit, now generally called a "cofidec circuit", interposed between the analog parts of the telephone set (mainly the loudspeaker and the microphone) and the digital parts (designed to be connected to the telephone line).
A cofidec circuit is more precisely a coder-decoder with filtering circuits to limit the band of frequencies transmitted on the line. The coder is a logarithmic (or substantially logarithmic) compression coder, that is, it converts analog signals into digital signals but does so in assigning them an attenuation coefficient that is all the smaller as the amplitude of the signals is great. The reason for this logarithmic compression is the need to improve the signal-to-noise ratio of the small signals. The decoder establishes a reverse digital-analog coding to re-establish the levels of amplitude of the originally emitted signal.
The invention more particularly concerns telephone sets with an amplified listening function (namely sets having a fairly high-powered external loudspeaker so that people in a room can hear the speech signals received). The invention also concerns, especially, telephone sets having a so-called "hands free" function, namely sets that can work without a handset. These sets have, on the body of the set itself and not on a movable handset, firstly an amplified loudspeaker for listening and, secondly, a microphone that is sensitive enough to collect the speech signals emitted by a person who is at a certain distance for example, at a distance of one meter or several meters) from the set.
Sets with amplified listening and, all the more so, "hands free" sets have a problem that is difficult to resolve: this is the problem of removing the "Larsen" effect, namely removing the risk that the circuits of the set will go into oscillation, with the a strident noise being produced in the loudspeaker.
In effect, whenever there is high-powered loudspeaker, there is a risk that the sound signal from the loudspeaker will be re-injected into the microphone, setting up a feedback loop with a gain that may be greater than one. This risk is clearly very high in "hands free" telephones where the microphone (or microphone amplifier) is more sensitive than that of a standard handset.
The general structure of the circuits of a digital telephone set with a "hands free" function is shown in FIG. 1.
The microphone is designated by the reference MIC, the loudspeaker by the reference HP and the telephone line by the reference L.
A cofidec circuit is represented, on the whole, in two parts which are a digital-analog coder COD and a decoder DEC with a conversion function that is complementary to the function of the coder.
An interface circuit IF is interposed between the digital input/output side of the cofidec and the telephone line L. Its function is to apply the signal that is present at a digital output Snm of the cofidec to the telephone line. This signal corresponds to a speech signal emitted by the microphone. It also has the function of receiving, from the line, a signal emitted from the other end of the line, and of applying it to a digital input Enm of the cofidec.
Finally, a circuit ML fulfilling the "hands free" function and, notably, the function of removing the Larsen effect, is interposed between the analog input/output side of the cofidec and the analog elements such as the microphone and the loudspeaker. This circuit is connected, firstly, to an analog input Ean of the cofidec to apply the analog signals coming from the microphone to it and, secondly, to an analog output San of the cofidec to receive analog signals coming from the line and transmit them to the loudspeaker.
The circuit fulfilling the "hands free" functions and, notably, the anti-Larsen function is represented in greater detail in FIG. 2.
It comprises, essentially:
A first variable gain amplifier AMP1, connected on its input side to the microphone MIC and connected on its output side to the analog input Ean of the cofidec;
A second variable gain amplifier AMP2 connected on its input side to the analog output San of the cofidec and connected on its output side to the loudspeaker;
And signal processing circuits designed to analyze the signals emitted by the microphone towards the telephone line and the signals received from the line going to the loudspeaker, to provide for the anti-Larsen effect.
Before giving a detailed explanation of the circuit of FIG. 2, we shall briefly indicate what this circuit should do.
There is a main need, in a telephone conversation, to determine the speaking side and the listening side; For the telephone set of the speaking side, the gain of the microphone has to be increased (i.e. there has to be an increase in the gain of the amplifier AMP1) and the gain of the loudspeaker has to be reduced (i.e. there has to be a reduction in the gain of the amplifier AMP2). For the set of the listening side it is necessary, on the contrary, to increase the gain of the loudspeaker (through action on AMP2) and to reduce the gain of the microphone (through action on AMP1). But, in both cases, the total loop gain of the following chain has to be kept below one: the microphone MIC, the amplifiers, the crosstalk (unwanted or deliberate) which makes a portion of the signal return towards the loudspeaker, and the acoustic coupling between the loudspeaker and the microphone.
Thus, the Larsen effect is prevented at the same time as the microphone and loudspeaker gains are adjusted at each instant, as a function of the conversation in progress.
Another parameter to be taken into consideration is the ambient noise picked up by the microphone. This ambient noise should not be taken for speech and should not put the gain controls into the reverse of the required state.
The role of the processing circuits of FIG. 2, therefore, is to assess whether the emitted signal and the received signal are a speech signal or a noise signal, to reduce the transmission gain of the weakest of the two signals and to increase the transmission gain of the strongest signal, unless the strongest signal is an ambient noise signal.
The gain transition should therefore be done lightly to prevent sound "clicks" that are audible on the line. The transition should take place quickly when one side starts speaking, and slowly when this side stops speaking. Other problems have to be taken into account (for example, a speaker who interrupts the other one), but these problems are unrelated to the present invention and shall, therefore, not be referred to herein.
The circuits of FIG. 2 therefore include different blocks designed to make the above-mentioned comparisons and assessments.
The basic block is a signal envelope detector that assesses the mean level of the (emitted or received) signal. It receives the signal for which it is desired to assess the level, and it gives a signal that represents the shape of the slow, mean variations of this signal, the fast variations being removed.
A first envelope detector DE1 has an input connected to the microphone to analyze the emitted signal, and a second envelope detector DE2 has an input connected to the analog output of the cofidec to analyze the received signal.
The outputs of the two detectors are applied to the inputs of a comparator COMP that determines the signal having the highest mean level.
Besides, these outputs can each be applied to the input of a detector, DB1 and DB2 respectively, the role of which is to determine whether the analyzed signal is rather a noise signal or rather a speech signal. Without going into the details, the noise detectors DB1 and DB2 detect the peaks of the envelope signal at output of the envelope detectors and compare them with the envelope signal. If the instantaneous envelope becomes greater than a certain quantity at the last peak detected, it is a speech signal. If not, it is related to ambient noise (the particular feature of which is that of being fairly stable).
The outputs of the comparator COMP and the noise detectors DB1 abd DB2 are applied to a control circuit CD which controls the gains of the amplifiers AMP1 and AMP2 as a function of the results of analysis of the signals that are emitted and received.
In the prior art, the envelope detector is made with a circuit such as the one shown in FIG. 3.
It is a logarithmic gain detector, i.e. it gives a signal representing the mean value of the received signal, but with a logarithmic scale. The small signals are more amplified (or less attenuated) than the big ones. This enables an accurate comparison to be made between the emitted and received signal levels, both for the big signals and for the small signals, without the signal-to-noise ratio's being excessively high for the small signals and without any risk of saturation for the big signals.
The prior art logarithmic envelope detector essentially comprises a logarithmic gain amplifier 10 receiving the analog signal, the level of which has to be controlled. This amplifier is followed by a full wave rectifier 12 when the input signal has positive and negative half cycles. The rectifier is followed by a smoothing RC integrator 14, the time constant of which is chosen to make the fast variations in signals disappear and to preserve the slow variations of the envelope.
The output of the detector is taken at the output of the RC integrator.
The logarithmic amplifier is an operational amplifier looped between its output and its input by two diodes in parallel, upside down with respect to each other. Since the diodes have a logarithmic current/voltage curve when they are in direct mode and since, at any instant, at least one of the diodes is in direct mode, the amplifier 10 has a logarithmic amplification coefficient, the amplification being far greater for the small signals than for the big ones.
FIG. 4 shows, as an example, a waveform of an analog signal received at the input of the envelope detector of FIG. 3 (line A). The line B represents the logarithmically compressed signal, at output of the amplifier 10. The range of variation of the signals has been reduced. The amplitude ratio between the small signals and the big ones is considerably smaller than on the line A. The line C represents the signal at output from the rectifier 12. Finally, the line D represents the output signal of the RC integrator 14. The fast variations of the input signal have disappeared. All that remain are the slow variations representing the envelope of the input analog signal or its mean level, but with a logarithmic scale.
In the telephone sets proposed up till now, the circuit of FIG. 1 is made by means of several different integrated circuits. The cofidec is generally made with an integrated circuit using MOS (CMOS or NMOS) technology which lends itself well to the making of conversion and filtering circuits. However, the circuit fulfilling the "hands free" functions is achieved by means of an integrated circuit using bipolar technology.
For, it is not possible to integrate two diodes in parallel and upside down with respect to each other into MOS technology.
Furthermore, whether it is MOS technology or bipolar technology, the circuit of FIG. 3 requires high value capacitances for the smoothing, and these capacitances cannot be integrated.
The present invention proposes a new circuit for telephone sets, enabling the envelope detector (and hence the entire "hands free" function) to be integrated into the same integrated circuit as the cofidecs, even if this integrated circuit is made by means of MOS technology. Another aim of the invention is the elimination of the external capacitances necessary for the signal envelope detection function.